1. Field of the Invention
The present invention relates to sensor control circuits suitable for airbag devices.
2. Description of the Prior Art
While the number of passengers suffering injuries from automobile collision increases with the increasing use of vehicles, airbag devices have been put into practice which are mounted on the steering wheel or instrument panel to instantly inflate the airbag upon collision to prevent the passenger from crashing against the steering wheel or windshield glass. The airbag devices are composed of an airbag, an actuator, and a sensor control circuit.
The actuator is composed mainly of a heater which quickly converts electric current into heat and an explosive which is blown up by the heat to ignite a gas producing material. The produced gases instantly inflate the airbag.
The sensor control circuit is composed of a sensor for detecting the acceleration of a vehicle, a processing unit for determining whether the acceleration exceeds the threshold value because of a rapid deceleration resulting from a collision or urgent braking, and a switching unit driven by an emergency signal outputted from the processing unit when the acceleration exceeds the threshold value to conduct current to the heater of the actuator.
The processing unit is composed of a processing device such as a microcomputer. In order to increase the reliability of airbag devices, there are provided a pair of systems each consisting of the sensor and processing section. In addition, a pair of switching sections are provided across the heater so that only when both of the detecting systems detect an abrupt stop, both of the switching sections are turned on to energize the heater. In this way, the heater is prevented from being energized by a malfunction of the detecting systems.
However, the processing unit composed of a microcomputer, for example, can be runaway because of disturbance noise. When this happens, even if the sensors do not detect any abrupt stop, both of the switching sections can be turned on by the runaway processing unit to conduct current to the heater. Consequently, the explosive is blown up to cause the gas producing material to produce gases, which instantly inflate the airbag. As a result, the airbag blocks the passenger's view, creating a very dangerous driving condition. In addition, the blown airbag is wasted.